1. Field of the Invention
The present invention relates to a fuel pump that draws in a fuel, increases the pressure thereof, and discharges the pressurized fuel.
2. Description of the Related Art
A known fuel pump generally comprises a casing and an impeller rotatably disposed within the casing. A first group of concavities is formed in a lower surface of the impeller. A second group of concavities is formed in an upper surface. The concavities are repeated in a circumferential direction. A first groove is formed in a first inner surface of the casing in an area that faces the first group of concavities of the impeller. A second groove is formed in a second inner surface of the casing in an area that faces the second group of concavities of the impeller. The first and second grooves extend in a circumference direction from an upstream end to a downstream end, respectively. A pump channel is formed inside the casing by the groups of concavities of the impeller and the grooves of the casing. An intake hole and a discharge hole are formed in the casing. The intake hole links the upstream end of the pump channel and the exterior of the casing. The discharge hole links the downstream end of the pump channel and the exterior of the casing. When the impeller rotates, the fuel is drawn from the intake hole into the pump channel. The fuel drawn into the pump channel flows from the upstream end to the downstream end of the pump channel, while the pressure thereof is increased. The pressurized fuel is discharged to the outside of the casing via the discharge hole.
In this known fuel pump, a clearance is provided between the casing and the impeller. Where the clearance is large, the fuel in the pump channel easily leaks from the pump channel to the clearance and high pump efficiency cannot be obtained. Therefore, in order to obtain high pump efficiency, it is necessary to decrease the clearance and reduce fuel leakage from the pump channel. However, if the clearance is too small, the casing and the impeller come into surface contact and sliding resistance increases, thereby decreasing pump efficiency. Thus, because of this trade-off relationship between the reduction in sliding resistance and reduction in fuel leakage, a technology is required that can realize the two at the same time.
Japanese Laid-open Patent Application Publication No. 6-213195 discloses a fuel pump in order to solve this problem. In this fuel pump, a plurality of concave portions is formed in the entire inner surface of the casing. The concave portions are formed as dots or grooves, and the concave portions are separated from each other. By forming a plurality of concave portions in the inner surface of the casing, the surface tension of the fuel increases and the adhesion force of the fuel inside the clearance between the casing and the impeller increases. As a result, sealing capacity can be improved without unnecessarily reducing the clearance between the casing and the impeller. Thus, with this fuel pump, both the reduction of fuel leakage and the reduction of sliding resistance can be realized.